School of BioSciences - Theses

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    From little things big things grow - savanna burning, suppressed trees and escape from the fire trap in Australian mesic savannas
    Freeman, Michelle Elizabeth ( 2017)
    This thesis explores growth responses and strategies of fire-suppressed trees in mesic savannas. Frequent fires are common to savannas globally, and cause most savanna trees to remain trapped as resprouts in the understorey by a cycle of topkill, where all above-ground parts of the plant are killed, followed by resprouting. Escape of suppressed resprouts from this fire trap is reflected in savanna tree community structure and composition. In this thesis, I contribute to the growing body of work from across the savanna biome that seeks to unravel the different effects of fire, competition and species growth strategies as mechanisms driving savanna tree communities. This question is fundamental to understanding what limits tree biomass in savannas and to predicting effects of different fire regimes in both the short term and in future climate scenarios. Despite much argument and modelling, mechanistic drivers of mesic savannas remain topics of conjecture, in part due to historical, environmental and species trait differences between continents. I collected the data used throughout this thesis within the Tiwi Carbon Study, a nine-year long fire experiment that aimed to provide accounts of carbon stored in soils, live vegetation and dead biomass under different fire regimes. The carbon economy is becoming a significant economic contributor to Aboriginal communities across remote northern Australia, with associated human benefits of social empowerment, wellbeing and connection to traditional practices. The increased focus on active management of northern Australian savannas for carbon sequestration and emissions abatement within a carbon market provides a human perspective to the ecological focus of my thesis. Within this context, there is a renewed imperative to understand what limits trees in savannas to anticipate effects of changes to fire regimes on carbon stocks and biodiversity. Using individual-level data I collected for 11 common resprouting savanna tree species subjected to different fire regimes on the Tiwi Islands in monsoonal northern Australia, I: (1) develop a theoretic framework that describes persistence and escape of suppressed resprouts subjected to frequent fire; (2) develop novel methods for estimating species and fire-specific escape heights; (3) model resprout growth and escape from the fire trap as mediated by fire and competition; (4) define sapling growth strategies based on functional and architectural traits that may influence escape potential; and, (5) demonstrate the effects of varying fire frequencies on savanna structure and composition. I found that the likelihood of escape from the fire trap is context-specific and related to differences in fire intensity, species traits and topkill-avoidance. Fire promotes fast growth of trees compared to fire exclusion, which may promote higher escape rates over shorter timeframes. However, less frequent fire leads to increased midstorey densities overall, thus affecting stand structure. In Australian savannas, eucalypts receive particular attention because of their canopy dominance, but I found minimal evidence of distinctly different growth responses between eucalypt and non-eucalypt resprouts that might explain this. Fine-scale environmental variation and individual species characteristics must be considered for robust estimates of escape from the fire trap. My research further implicates non-fire disturbances and different reproductive strategies as potentially illuminating drivers of different species responses – important topics for future research.